Thousands of air passengers have been hit by lengthy delays amid security tensions in the wake of the botched transatlantic airliner bomb plot.
Amid anxious scenes at the same US airport where a former London student allegedly tried to ignite his device on Christmas Day, a second Nigerian man was arrested on the same route on Sunday.
A security official at Detroit airport confirmed later he posed no security risk to the plane. He was a businessman who had become "verbally disruptive" after falling ill during the flight, the officer said.
In Britain, passengers waiting to fly to America for New Year were left waiting up to three-and-a-half hours while urgent investigation took place to find out how Umar Farouk Abdulmutallab - who was known to US and British authorities - was able to take explosives on board a transatlantic airliner carrying 278 passengers.
US President Barack Obama has ordered a fresh review of screening processes after it emerged authorities had been warned about Abdulmutallab's extremist views by his own millionaire father.
Meanwhile, Scotland Yard detectives continued to search the 23-year-old's luxury apartment in a central London mansion block as it emerged an al Qaida operative in Yemen threatened the US and said "we are carrying a bomb" in a video posted online four days before the attack.
Amid panicked scenes in Detroit on Sunday, the flight crew became concerned after the second man - also Nigerian - became sick and spent about an hour locked in the bathroom.
"This raised concerns so an alert was raised," FBI spokeswoman Sandra Berchtold said. "The investigation shows that this was a non-serious incident."
Among the delays caused by the initial plot, one flight from London's Heathrow to New York's Newark airport was late leaving by more than three hours, although most delays were less severe.
People flying across the Atlantic were advised to arrive early and carry just one piece of hand luggage.